The present invention relates to a method for treating aluminum in a furnace, wherein at least one material containing aluminum and optionally one or more salts and/or slag and/or recycled dross is introduced into the furnace, this material is melted by the input of heat using at least one burner supplied with oxidizer and with fuel, in order to obtain molten aluminum optionally covered with a slag comprising alumina in particular, and the concentration of carbon monoxide (COm) and/or of hydrogen (H2m) in the furnace atmosphere or in the flue gases exiting from the furnace is measured.
In the field of the melting of secondary aluminum, this is carried out in a rotary furnace or a reverberating furnace. Although this melting method may be continuous, the melting is usually carried out in batches: the materials are charged into the furnace in one or more successive cycles before the molten metal is poured to its place of use. For this purpose, the molten metal must have a temperature of about 740° C. Above 750° C., the oxidation rate of the molten aluminum rises considerably, almost exponentially.
During a melting cycle, one can first distinguish the initial period, when the materials are solid, allowing the absorption of a large quantity of heat, the aluminum melting at about 660° C.
Irrespective of the type of furnace used, the existence of a slag or “dross” is observed on the liquid metal surface, said slag or dross being a mixture of salts (if salts are used), aluminum oxide and aluminum trapped in the oxide.
This slag or dross represents a quantity of lost or oxidized metal also called “loss on ignition” which represents a non-negligible loss of material for the aluminum producer, and which should be minimized to increase the profitability of the melting method. To reduce this oxidation, it is known how to maintain the temperature of the aluminum melt at a value lower than about 750° C. However, this method is empirical, because hot spots may appear on the surface, causing local oxidation.
Other known solutions attempt to prevent oxidation by reducing the contact of the metal surface with an oxidant.
Thus, document JP 53-227706 proposes to use the measurement of the CO and H2 contents in the flue gases to ensure that, in a melting furnace for nonferrous metal, the burners installed operate in substoichiometric mode in a range of ratios of oxidant flow rate to fuel flow rate of between 95 and 100%.
Document EP 962 540 describes a combustion method for melting a metal in a furnace, in which an oxygen-rich gas is sent to the furnace, above the flame of a burner, in contact with the liquid metal.
The burner, operating in substoichiometric mode, produces a reducing flame forming a shield between the oxygen-rich gas and the molten metal surface.
Document U.S. Pat. No. 5,563,903 describes a method in which an inert or reducing gas forms a shield between the aluminum melt surface and a combustion zone located in the upper part of the furnace.
Document U.S. Pat. No. 3,759,702 describes a method in which the melting initially takes place in the open air, with a burner moving above the surface of the materials to be melted. The burner flame is slightly substoichiometric, hence reducing.
All the methods yield approximate results and are applied throughout the melting duration and not only when a risk of oxidation of the aluminum exists, because this knowledge has so far been lacking to a person skilled in the art.